A valve suitable for abrasive fluids such as oil field drilling mud comprises a valve body and a corresponding valve seat, with certain valve bodies incorporating an elastomeric seal within a peripheral seal retention groove. Such a valve is usually mounted in the fluid end of a pump incorporating positive displacement pistons or plungers in multiple cylinders. Such valves may be stem-guided full-open designs (i.e., having a top guide stem and a bottom crow-foot guide on the valve body) or web-seat, stem-guided designs (i.e., having top and bottom guide stems on the valve body). Note that stem-guided full-open designs are also commonly known as full-open seat wing-guided designs; the former term will be used herein for convenience. However named, the above valves are expensive to manufacture, especially the moving portion or valve body. Besides requiring finish machining to close tolerances for adequate sealing, such valve bodies must be made strong enough to resist significant distortion under load with resultant leaks and fatigue failures. Prior efforts to reduce distortion under load by strengthening such valve bodies have generally resulted in higher cost and/or heavier designs which exacerbate sealing problems and/or increase the stress of impact loading on components of the valve assembly.
Commercially important design improvements necessarily reflect the fact that certain mud pump valve body and seat dimensions are effectively limited by industry practices and American Petroleum Institute (API) Standards. For example, the web-seat, stem-guided designs favored for mud pump valves are commonly made compatible with the industry benchmark widely known as the TRW Mission 4-web seat, which determines many valve dimensions. Further, API Standards determine the envelope into which valve bodies and seats must fit to promote interchangeability in the field.
Given these constraints, attempts to reduce fatigue failures and/or improve valve performance have led to “improved” designs which are more expensive to manufacture and have different failure modes than earlier versions. For example, valve bodies with circular “Channel-Beam” sections may incorporate a forged bowl shape as seen, for example, in FIG. 1 of U.S. Pat. No. 5,249,600, the entire patent being incorporated herein by reference. This forged valve body has exceptional stiffness and strength.
But such valve bodies have several disadvantages in manufacture and use. First, rough valve body forgings in the Channel-Beam shape require substantial material removal in finish machining of the integral seal retention groove. Second, an elastomeric seal snapped into the seal retention groove may not fully seat around the entire valve body, causing an out-of-round condition that can result in early valve failure. Third, certain portions of the valve body may be made relatively thin to reduce weight, but such thin portions require particular care during heat treatment to avoid excessive brittleness. Avoidance of thin portions, on the other hand, imposes weight penalties that result in greater impact loading. Similar disadvantages are generally present in other Channel-Beam designs, such as those described in U.S. Pat. Nos. 3,191,617; 3,202,178; 3,742,976; 4,180,097; 5,345,965; and 5,431,186, all incorporated herein by reference.
Notwithstanding their relatively high cost, however, valve bodies having an integral seal retention groove (i.e., a seal retention groove having no removable seal retention plate or other analogous removable structural member) such as the one-piece Channel-Beam design have gained limited industry acceptance. Their high strength and stiffness effectively counter valve body distortion about one or more axes radiating perpendicularly from the valve body's longitudinal axis of symmetry (radial axes). Distortion about radial axes is particularly a problem on valve bodies that mate with web seats. Cyclical high pressure applied to a valve body when it is sealed against a web seat is especially damaging, tending to repeatedly force portions of the valve body into the spaces between the seat webs. The periphery of the disc-shaped area of the valve body (commonly called the flange) then tends to wrinkle like a cupcake paper, the number of wrinkles being equal to the number of seat webs.
On multi-piece valve bodies (i.e., valve bodies having a removable seal retention plate), cyclic loading as described above induces fatigue that can lead to further distortion and/or failure of the valve flange. Countering such distortion by simply making the flange thicker increases total valve body weight, which in turn increases wear due to higher impact loading of both the valve body and seat. Valve bodies of Channel-Beam design minimize such distortion in part through their inherent rigidity and strength, but they weigh even more than comparable multi-piece valve bodies and so suffer the disadvantage of higher impact loads in use.
Another important disadvantage of the Channel-Beam design, as noted above, relates to seating of the seal insert. Channel-Beam valve bodies generally incorporate an elastomeric seal insert that snaps into its peripheral seal retention groove. A typical “snap-on” seal insert comprises a portion of a toroidal structure such as a plastic or rubber ring that is sized to fit tightly, and thus sealingly, in the peripheral seal retention groove. When properly fitted, the elastomeric seal mates tightly with a corresponding valve seat even though the valve body is slightly distorted and even if small particles carried by the pumped fluid may be trapped between sealing surfaces. Practical advantages of such a seal insert include extended valve life and improved valve performance, but proper fitting and sealing of the elastomeric ring on a valve body is often difficult in the field.
Another disadvantage of Channel-Beam valves is the relatively high manufacturing cost of the valve bodies themselves. They are expensive to manufacture because the forgings from which they are machined are not near-net-shape. Significant machining time is needed to remove excess material from the seal retention groove (the Channel). Further, because of their characteristic shape, Channel-Beam valve bodies require longer (non-standard) springs to accommodate the extra depth of the bowl.
Some of the above problems associated with high machining and materials costs, as well as those associated with seal movement and/or out-of-round seal placement, are reduced in valve bodies which incorporate a separate (removable) seal retention plate which commonly screws or bolts to the valve body to form at least part of one wall of a seal retention groove. Separate seal retention plates can be forged to near-net-shape, and they reduce the time required to correctly replace toroidal sealing rings. But they also raise valve fabrication costs and impose use restrictions. For example, they add excess weight to the moving valve body, aggravating impact loading stress. And a removable seal retention plate must be handled separately from the remainder of the valve body during manufacturing. Additionally, special skills and tools are required for proper assembly of a retention plate and seal ring on a valve body. Finally, the threads often used to secure a retention plate to a valve body are both expensive to machine and, because portions of the threads are relatively thin, they demand special protection during heat treatment. Nevertheless, removable seal retention plates are commonly used because such a plate, as well as the valve body to which it is attached in use, can be forged to a “near-net-shape” which requires relatively little finish machining to achieve a desired final shape.
Unfortunately then, even though forged valve bodies having integral seal retention groves, as in the Channel-Beam design, are inherently stronger than designs requiring a removable seal retention plate, they are generally heavier, more expensive to make, and prone to failure due to seal movement and/or out-of-round seals. What is needed is a valve body having strength and rigidity comparable to that of the Channel-Beam design without the disadvantages of high production costs, seal movement and/or out-of-round seal placement.
Attempts to overcome the cost disadvantage of forged valve bodies having integral seal retention grooves have included elimination of forgings altogether, substituting cast valve bodies instead. Though such castings may be produced to near-net-shape and thereby reduce machining costs, the generally higher cost of the casting process itself, compared to forging, has substantially eliminated any hoped-for reduction in overall cost. Additionally, cast valve bodies generally have lower impact strength compared to similarly shaped forgings. Thus, there is a need for a relatively light weight forged valve body incorporating the strength advantages of an integral seal retention groove and the efficiencies of initial formation to near-net-shape.